Transition-State Charge Transfer Reveals Electrophilic, Ambiphilic, and Nucleophilic Carbon−Hydrogen Bond Activation

Abstract

Absolutely localized molecular orbital energy decomposition analysis of C−H activation transition states (TSs), including Pt, Au, Ir, Ru, W, Sc, and Re metal centers, shows an electrophilic, ambiphilic, and nucleophilic charge transfer (CT) continuum irrespective of the bonding paradigm (oxidative addition, σ-bond metathesis, oxidative hydrogen migration, 1,2-substitution). Pt(II) insertion and Au(III) substitution TSs are highly electrophilic and dominated by C−H bond to metal/ligand orbital stabilization, while Ir−X and Ru−X (X = R, NH₂, OR, or BOR₂) substitution TSs are ambiphilic in nature. In this ambiphilic activation regime, an increase in one direction of CT typically leads to a decrease in the reverse direction. Comparison of Tp(CO)Ru−OH and Tp(CO)Ru−NH₂ complexes showed no evidence for the classic d_π−p_π repulsion model. Complexes such as and Cp(CO)₂W−B(OR)₂, (PNP)Ir(I), Cp₂ScMe, and (acac-κO,κO)₂Re(III)−OH were found to mediate nucleophilic C−H activation, where the CT is dominated by the metal/ligand orbital to C−H antibonding orbital interaction. This CT continuum ultimately affects the metal−alkyl intermediate polarization and possible functionalization reactions. This analysis will impact the design of new activation reactions and stimulate the discovery of more nucleophilic activation complexes.